Films containing voids or pores have been made in the past by a variety of techniques from polymers containing solid substances dispersed therein. Sometimes voids were obtained by dissolving or leaching out those dispersed materials (or fillers), and sometimes voids formed when the filled polymeric material was stretched. In most porous filled polymer films, the filler was shown to nucleate the void formation upon stretching or leaching out see, for example, U.S. Pat. Nos. 3,738,904 (Ikeda et al.), 3,783,088 (Yohiyasu et al.) and 4,176,148 (Magder et al.).
Porous films produced heretofore by stretching filled polymer compositions were stiff, low tear strength products, irrespective of the type of matrix polymer used. Pore formation nearly always depended on propagation of cracks in the matrix polymer, and as a result, such films were fabricated with little or no control over pore size, making it impossible to achieve simultaneously high porosity and small pore size, which characterize microporous films made by more expensive methods from more expensive materials. Because of their stiffness, low strength, and lack of pore size control, those prior art stretched filled polymer films had a rather limited utility.
It thus is desirable to be able to produce soft, flexible, high strength, microporous films having high porosity by stretching filled polymeric materials.